Sealing arrangement in turbine machinery

ABSTRACT

A sealing arrangement in a steam turbine is deployed in a position where it can restrict flow of steam along a flowpath in a gap between two bodies of the turbine rotatable relative each other, the flowpath proving communication between a space of relatively high pressure and another of relatively low pressure. The sealing arrangement comprises a leaf seal mounted on one of the bodies and sealingly engaged with the other. Two or more leaf seals can be juxtaposed with each other, without need for labyrinth seals, in a steam turbine application, without deleterious effects experienced in use of brush seals in similar circumstances. The sealing arrangement can include segments displaceable from a sealing position to increase clearance during start-up of a turbomachine, thereby taking account of potential vibration problems as the turbine rotor passes through natural frequencies.

The present invention relates to a sealing arrangement in turbinemachinery, particularly in steam turbine machinery.

Seals are employed in steam turbine machinery at points where a surfaceof a rotating body of a machine (the rotor) opposes a surface of astationary part of the machine (the stator). Seals are used to restrictthe passage of fluid from a space of relatively high pressure to a spaceof relatively low pressure.

Seals for steam turbine machinery have historically been in the form ofso-called “labyrinth” seals. A labyrinth seal, formed in a flow pathwaybetween a rotor and a stator comprises one or more formations onsurfaces of the rotor and/or stator, to present obstructions to the flowof fluid in the flow pathway. These obstructions constrict thecross-sectional area of the flow pathway so that the fluid is forced toaccelerate and thus become of relatively low pressure, and then todecelerate very rapidly as it passes through the labyrinth seal. Thisrapid deceleration of the fluid as it exits the constriction causesuncontrolled expansion of the fluid, which induces energy losses whichare effective at presenting upstream obstruction to fluid flow. However,it is appreciated widely that labyrinth seals are only partiallyeffective, and improvements thereto are desired.

To provide improved performance, brush seals have been introduced to actinstead of or alongside labyrinth seals. A brush seal comprises abacking plate into which are embedded a very large plurality of pliablewire or fibre filaments. A brush seal also operates to provide atortuous path to the progress of gas or other fluid through the seals.In operation, a brush seal is about 90% more effective than a labyrinthseal in holding a pressure drop between two spaces in a turbo machine.

In some circumstances, therefore, brush seals provide an effective sealin turbo machines where the gap between the rotor and stator is in theorder of 1 mm. However, in gas turbines, the gap between the rotor andstator components can vary widely. In particular, thermal, gyroscopicand other mechanical effects can cause variation in the gap between therotor and stator, and brush seals have been found to be effective atdealing with this.

One observation has been made that the maximum pressure drop that can becarried by the seal is inversely proportional to the square of the gap.Thus, by increasing the gap to, say, 3 mm, a seal with the same bristlesas the seal described above would only be able to accommodate a maximumpressure drop one ninth of that which can be accommodated by the sealdescribed above with the 1 mm gap.

It has been found through experience that the construction of a brushseal capable of withstanding these gaps would require fibres so thickand strong that their compliance would be severely limited. This wouldraise the risk of damage to the brush, and to wear within the turbomachine. Thus, the need has been identified to devise a seal capable ofovercoming this problem.

However, in steam turbines different technical problems are presentedthat are of greater significance than the above problems relating to theoperation of gas turbines. Indeed, the above problems may not beexhibited at all in steam turbines. Steam turbines are generally muchlarger than gas turbines and they operate at relatively lowertemperatures. Further, rotational speeds of rotors in steam turbines canbe lower than in a gas turbine.

For these reasons steam turbines are perceived to operate in morecontrolled conditions than those under which a gas turbine operates.Because of these less extreme conditions, rotor-stator gap is not highlyvariable—temperature expansion of moving parts in a steam turbine islikely to be less significant and gyroscopic effects are likely to begreatly reduced. In these circumstances, a 1 mm clearance between therotor and stator is usually sufficient and can usually be relied upon toremain relatively constant, at least to the extent that ‘hard on hard’contact (contact of the rotor and stator surfaces themselves) can berealistically assumed not to be a risk.

On the other hand, pressure drops experienced in steam turbines areoften significantly higher than in gas turbine technology. For example,a typical pressure in a high-pressure cylinder of a steam turbine couldbe as much as 200 bar, while exhaust pressure might be in the region of80 bar. In the past, labyrinth seals have been used to control thissubstantial pressure drop. However, it is appreciated that the length offlow path presented by such a labyrinth must be significant to providesignificant constriction on the flow path, and this will addsignificantly to the axial length of a turbo machine. This couldsignificantly increase the size of the machine in relation to itspurpose and could impact on the positioning of other machines to whichthe turbo machine should be connected in installation.

Brush seals have been presented as possible solutions to this problem,in that they can control such a pressure drop over a shorter axiallength than a labyrinth seal. However, in practice, brush seals havebeen found to be inappropriate in steam turbine applications. Inparticular, it has been found that brush seals are inappropriate insituations where very high-pressure steam and high swirl areencountered. The swirl kinetic energy can cause significant disruptionof the brush elements of the brush seal, which can lead to failure ofthe seal. Further, this swirling of the inlet steam can, over time, leadto fatigue related damage to the filaments of the brush.

In addition, high-pressure steam, such as is encountered in steamturbines, can carry significant particulate matter, such as detritusfrom the interior of boiler apparatus used for generating the steam.Such particulate matter can impact with the brush seal so as to causesubstantial damage to the brush filaments.

Thus, it would be desirable to be able to provide a seal for a steamturbine machine capable of operating in conditions of swirling intakeand in which the risk of damage to the seal by steam borne particulatesis reduced.

In accordance with one aspect of the invention, a steam turbo machinecomprises a stator and a rotor rotatable with respect to the stator,opposing circumferential surfaces of the stator and rotor definingbetween them a plurality of gaps capable of fluid communication, thegaps being sealed by sealing arrangements, wherein at least one of thesealing arrangements is a leaf seal.

By using a leaf seal in a steam turbine in preference to a labyrinthseal or a brush seal, problems previously exhibited through the use of alabyrinth seal (such as the extreme length of such a labyrinth seal) ora brush seal (susceptibility to damage through swirl and steam borneparticles) can be reduced.

In a balance piston seal environment, pressure differences between thehigh-pressure area and the low-pressure area can be extreme. Even in thecase of a leaf seal, it can be difficult to arrange for a sealing meansto support the pressure drop required for effective operation of a steamturbine. Therefore, it has long been understood that placing two or moreseals in series may be an effective way of producing an appropriateseal.

However, it has been found that brush seals, placed in series are lesseffective than anticipated, as the flow of steam through the first sealproduces unpredictable flow effects in the space between the two seals,affecting the sealing effectiveness of the second seal. This can have adeleterious impact on the efficiency and operation of brush seals inseries.

In the past, this problem has been solved by inclusion of labyrinthseals between brush seals in series; as noted above, labyrinth seals cancompromise the compactness of a sealing arrangement, which can have anegative impact on the effective arrangement of turbine machinery in aplant. The inclusion of a labyrinth seal can also sometimes compromisethe assembly of a sealing system. This has led to compromise in thelevel of compactness of a sealing arrangement comprising two or morebrush seals in series. Thus it is desirable to provide a sealingarrangement directed at addressing these problems, such that sealingeffectiveness can be improved without compromising machine compactness.

According to a further aspect of the invention, a turbomachine poweredwith a flow of steam defines a region of relatively high pressure and aregion of relatively low pressure, separated by a flow path between tworelatively rotatable members of the turbomachine, and a sealingarrangement operable to restrict the passage of steam in said flowpath,wherein said sealing arrangement comprises a series of leaf seals.

A surprising consequence of providing leaf seals in series is that theneed to provide labyrinth seals to stabilise flow patterns to maintainseal effectiveness is obviated.

Further aspects of the invention, advantages thereof, and problemssolved thereby will become apparent from the following description ofspecific embodiments of the invention accompanied by the appendeddrawings in which:

FIG. 1 is an axial cross-section through a steam turbine in accordancewith a specific embodiment of the invention;

FIG. 2 is an axial cross-section through a sealing arrangement in thesteam turbine of FIG. 1;

FIG. 3 is a cross-section through a line marked A-A in FIG. 2; and

FIG. 4 is a cross-section, in a similar orientation to that illustratedin FIG. 2, of a sealing arrangement according to a second embodiment ofthe invention.

FIG. 1 illustrates a steam turbine of substantially known construction,but fitted with seals in accordance with a preferred embodiment of theinvention. Thus, the turbine 10 comprises a cylindrical casing 12 and arotor 14 which includes a balance piston 16 and trunnions 18, 20 atupstream and downstream ends respectively.

An outer cylindrical surface 22 of the rotor 14 and an inner surface 24of the casing 12 define a turbine chamber 26. The turbine chamber isdefined such that it is radially narrower at the upstream end than thedownstream end, in accordance with usual turbine design practices.

Projecting from the rotor surface 22 and the casing surface 24 areseries of rotor blades 30 and stator blades 32 respectively.

An inlet manifold 34 is defined in the casing 12, in fluid communicationwith the upstream end of the turbine chamber 26. As indicated in FIG. 1,in operation the turbine 10 receives a flow of steam through this inletmanifold 34 at a pressure of up to 200 bar. An exhaust manifold 36 isdefined in the casing 12 and is in fluid communication with thedownstream end of the turbine chamber 26; in operation exhaust steamexits the turbine 10 through the exhaust manifold 36, as indicated inFIG. 1.

Upstream end glands 40 are located circumferentially around the upstreamend trunnion 18, to prevent escape of inlet gas from the gap formedbetween the trunnion 18 and the casing 12. Balance piston seals 42 areplaced circumferentially around the balance piston 16, to restrict flowof gas through the gap defined between the balance piston 16 and thecasing 12. The balance piston seals 42 can experience a pressure dropacross them of up to 120 bar in operation. This is because the inputhigh pressure steam introduced by inlet 34 can be input at a pressure ofup to 200 bar, whereas the pressure in the exhaust manifold 36 (withwhich the balance piston is in communication in order to providebalancing thrust to the rotor) may be at a pressure of the order ofmagnitude of 80 bar. Rotor blade seals 44 are provided in the gapsbetween the tips of the rotor blades 30 and the inner surface 24 of thecasing 12. Corresponding stator blade seals 46 are provided between thetips of the stator blade 32 and the outer surface 22 of the rotor 14.Provision of rotor blade seals 44 and stator blade seals 46 restrictflow of gas at the tips of the blades 30, 32, thereby increasing thetendency of steam in the turbine chamber 26 to be forced past thereaction surfaces of the turbine blades and thereby provide torque tothe rotor 14.

FIG. 2 illustrates a balance piston seal 42, which comprises a supportcasing 50 of substantially annular construction, mounted on the interiorsurface 24 of the casing 12.

The casing 50 defines a channel in which are loaded a plurality ofleaves 52. The leaves 52 are generally rectangular and extendlongitudinally of the channel and thus radially of the balance piston16. Laterally extending lugs 54 correspond with opposing surfaces 56 ofthe channel 50, so as to cause the leaves 52 to be retained in thechannel. The ends of the leaves 52 directed away from the retaining lugs54 extend longitudinally from the casing 50 and protrude beyond theradial extent of casing 50.

FIG. 3 shows in further detail the leaves 52 in relation to thedirection of rotation of the piston 16. The leaves 52 are shingled so asto engage against the surface of the balance piston 16 as it moves past.It is anticipated that the compliance of the leaves 52 will provide areturn force to provide the desired sealing effect, as the seal leaves52 engage the surface of the balance piston 16.

It has been found that a leaf seal in this form is capable of supportinga pressure drop of 120 bar, such as will be experienced in the positionof a balance piston in a steam turbine. Further, it will be appreciatedthat similar leaf seals can be placed at the point of inlet gland seal40, exhaust gland seal 48, and for rotor blade and stator blade seals44, 46. Further, the structural rigidity of leaves 52 is superior to abrush seal of similar size, and is thus able to withstand the impact ofa swirling inlet gas. In addition, leaves 52 will be less susceptible todamage as a result of impact from particles borne on the inlet steam.

FIG. 4 illustrates a sealing arrangement according to a secondembodiment of the invention. This sealing arrangement is shown as, forthe purposes of this example, a balance piston seal 42.

Two axially spaced, parallel, bearing piston leaf seals 42 are arrangedon the casing 12 and directed inwardly of the turbine casing 12, toengage sealingly with the balance piston 16. No labyrinth features areinterposed between the two leaf seals 42. The two leaf seals act intandem to provide adequate separation between an upstream zone and adownstream zone in the turbine, thereby affecting the seal required foreffective operation of the turbine 10.

By arranging leaf seals in this manner, previous problems related toefficiency of brush seals operated in series, together with theadditional axial length that would be required of the machine in whichtwo brush seals were placed in series, together with supposed labyrinthseal, can be avoided.

In either of the above-described embodiments, it would be desirable toarrange that the leaf seals 42 are detachable from the casing 12, toallow worn leaves 52 of the leaf seals to be replaced from time to time.

It would be appreciated that the present invention is not limited to itsapplication to steam turbines. Other machines, in which similarconditions of temperature and pressure arise, could be susceptible tothe implementation of the present invention. The present inventionessentially addresses problems related to sealing a flow path definedbetween two relatively rotatable parts of a machine, running at speedscomparable to those encountered in a steam turbine, wherein seal limitsflow of steam between a region of high pressure (a pressure that mightbe experienced at an inlet of a steam turbine) and a region of lowpressure (a pressure that might be experienced at an exhaust outlet of asteam turbine).

It will be appreciated further that other modifications and adaptations,which provide technical effects complementary to the technical effect ofthe present invention, can be provided in a steam turbo machine. Forexample, as per the disclosure of U.S. Pat. No. 4,436,311A, the sealingarrangement may be mounted on radially movable segments about the rotor,the segments being urged apart by circumferentially aligned springs toprovide for a larger rotor/stator gap at low rotational speeds and lowpressure to avoid ‘hard on hard’ contact occurring during a start upphase of a turbine. As described in that document, the turbine isparticularly susceptible to hard on hard contact during start up asturbine rotor may, as it accelerates rotationally, pass through speedsat which a natural frequency of the rotor is encountered. This can leadto vibration of the rotor and consequent risk of fouling against astator component that is positioned too close to the rotor.

Once the rotor has reached a driving speed, high-pressure steam is thenintroduced to the turbine. The segments of the sealing arrangement arearranged such that the reaction force applied by the steam to thesegments of the sealing arrangements is operable to urge the segmentstowards a sealing position against the rotor. Radial movement of thesegments towards the rotor may be limited by a captive formation withrespect to the turbine casing, to prevent the sealing arrangementsegments from fouling the rotor.

1. A steam turbo machine comprising a stator and a rotor rotatable withrespect to the stator, for receiving an inlet flow of steam for drivingthe rotor, wherein opposing circumferential surfaces of the stator androtor define between them a gap capable of enabling fluid communicationbetween a space of said machine of relatively high pressure in use to aspace of relatively low pressure in use, the gap being sealed by asealing arrangement, wherein the sealing arrangement is a leaf seal. 2.A machine according to claim 1 wherein said rotor includes a balancepiston, for maintaining a compensating force with respect to thereaction of the rotor against the inlet steam in operation, said balancepiston having a circumferential surface which defines, with an opposingcircumferential surface of the stator, said gap between a space which,on operation of the turbine, is at a relatively high pressure and aspace which, on operation of the turbine is at a relatively lowpressure, a sealing arrangement being interposed in said gap and whereinsaid sealing arrangement is a leaf seal.
 3. A machine according to claim1 wherein said relatively high pressure is a steam inlet pressure at asteam inlet of the steam turbine machine.
 4. A machine according toclaim 1 wherein said relatively low pressure is an exhaust pressure at asteam exhaust of said steam turbine machine.
 5. A machine according toclaim 1 wherein said rotor has an external circumferential surface, andsaid stator has an internal circumferential surface, developed aroundthe axis of rotation of said rotor, and a turbine chamber is definedbetween said surfaces, said rotor having a plurality of substantiallyradially extending turbine blades, configured to present a surfaceagainst which said inlet flow of steam reacts in use to generate atorque acting on said rotor relative said stator, each of said turbineblades extending radially to adjacent said inner surface of said stator,so as to define a gap in which a sealing arrangement is interposed, saidsealing arrangement being a leaf seal.
 6. A machine according to claim 5wherein said stator has a plurality of inwardly substantially radiallyextending turbine blades configured to direct, in operation of saidturbine, said inlet flow onto said turbine blades of said rotor to causesaid torque to be developed in use, wherein each of said stator turbineblades extend radially to adjacent said external surface of said rotorand thereby defining a gap in which a sealing arrangement is interposed,said sealing arrangement comprising a leaf seal.
 7. A machine accordingto claim 1 wherein the rotor comprises opposing and coaxial mountingmeans for cooperation with corresponding formations of said stator, saidmounting means and formations each having opposing adjacentcircumferential surfaces defining a gap in which a sealing arrangementis interposed, wherein said sealing arrangement is a leaf seal.
 8. Amachine according to claim 1 wherein said sealing arrangement comprisestwo leaf seals, interposed in said gap.
 9. A machine according to claim1 wherein the or each leaf seal comprises a plurality of leavesextending from one of the rotor and stator across said gap towards theother of the rotor and stator.
 10. A machine according to claim 1wherein said sealing arrangement comprises a plurality of supportsegments, circumferentially arranged about the gap, each support segmentpresenting a leaf seal in said gap, each segment being radiallydisplaceable in said gap, wherein said sealing arrangement furthercomprises resilient means for urging said segments into an initialposition displaced away from a sealing position, said segments arearranged to be urged into said sealing position by a reaction forcedeveloped in use through high pressure in said high pressure region ofsaid turbine machine.
 11. A method of sealing a gap in a steamturbomachine between a region of relatively high pressure and an area ofrelatively low pressure, between relatively rotatable members of saidturbomachine, said method comprising interposing a leaf seal in saidgap, mounting said seal on one of said members and directed sealingly ata surface defined on the other of said members.
 12. A steam turbinemachine substantially as described herein with reference to FIGS. 1 to 3of the accompanying drawings.
 13. A steam turbine machine substantiallyas described herein with reference to FIG. 4 of the accompanyingdrawings.
 14. A method of sealing a gap in a steam turbomachinesubstantially as described herein with reference to FIGS. 1 to 3 of theaccompanying drawings.
 15. A method of sealing a gap in a steamturbomachine substantially as described herein with reference to FIG. 4of the accompanying drawings.